lambda: forbid type params in SAM methods (IDEA-101553)

java/java-psi-api/src/com/intellij/psi/LambdaHighlightingUtil.java

@@ -32,7 +32,12 @@ public class LambdaHighlightingUtil {
     final List<MethodSignature> signatures = LambdaUtil.findFunctionCandidates(psiClass);
     if (signatures == null) return "Target type of a lambda conversion must be an interface";
     if (signatures.isEmpty()) return "No target method found";
-    return signatures.size() == 1 ? null : "Multiple non-overriding abstract methods found";
+    if (signatures.size() == 1) {
+      final MethodSignature functionalMethod = signatures.get(0);
+      if (functionalMethod.getTypeParameters().length > 0) return "Target method is generic";
+      return null;
+    }
+    return "Multiple non-overriding abstract methods found";
   }
 
   public static String checkReturnTypeCompatible(PsiLambdaExpression lambdaExpression, PsiType functionalInterfaceReturnType) {

java/java-tests/testSrc/com/intellij/codeInsight/daemon/lambda/FunctionalInterfaceTest.java

@@ -76,11 +76,11 @@ public class FunctionalInterfaceTest extends LightDaemonAnalyzerTestCase {
   }
   
   public void testMethodWithTypeParam() throws Exception {
-    doTestFunctionalInterface(null);
+    doTestFunctionalInterface("Target method is generic");
   }
   
   public void testTwoMethodsSameSignatureTypeParams() throws Exception {
-    doTestFunctionalInterface(null);
+    doTestFunctionalInterface("Target method is generic");
   }
 
   public void testAbstractClass() throws Exception {